1. Field of the Invention
The invention relates to a method for moving or driving an object, more particularly for controlling periodic motion of a reflector element such as e.g. a galvanometer mirror for incremental or "raster"-type deflection of a laser scanning beam, and to devices for implementing the methods.
2. Description of the Prior Art
In measuring and experimenting systems there is often the requirement to raster-scan a system being investigated (sample) as regards a specific quantity to be measured. In a microscope, for instance, a sample is to be scanned by a light signal by means of a movable deflection mirror. The deflection of the deflection mirror is required to obey a specific time pattern to which data acquisition is adapted. Another example is the scanning motion of field-forming elements (e.g. grids) for local resolving electron-spectroscopic analyzers.
The problems associated with signal control in generating scanning motion will now be discussed by way of a deflection mirror of a confocal laser fluorescence scanning microscope (abbreviated LSM) as an example.
For LSM line scanning the laser beam needs to be steered over an object at an angle interval line-by-line in a time profile as linear as possible. This is done as a rule by galvanometer mirrors which are subjected to a driver signal of the desired scan angle time profile.
Usally, for this purpose feedback galvanometer mirrors having an angle sensor (see E. H. K. Stelter in "Handbook of Biological Confocal Microscopy", Plenum Press, New York 1995, pages 139-154) are used. In a feedback circuit having analog amplification the sensor signal is compared to a command signal (reference variable input). Using a correspondingly generated positioning signal the mirror is subjected to the desired deflection. The command signal may be, for example a linear function. However, due to the roughly 0.5 to 2 ms transient phase of the mirrors in abrupt deflection scanning rates of only approximately 500 Hz are achieved, representing a limitation in real-time analysis.
To achieve higher scanning rates the mirrors are operated freely oscillating with no feedback for a predetermined frequency (for example for a resonant frequency) of the mirror. In this case deflecting the mirror is substantially sinusoidal. Data acquisition (width of the sensing intervals) is accordingly irregular which is a drawback in interpreting the data. This is why for data acquisition use is often made of only a small deflection range about zero where the sine function is more or less linear. This, however, has the drawback that only a fraction (e.g. 30%) of the working cycle of a full deflection period is available for data acquisition. In addition, complicated means of correction may be necessary after data acquisition to linearize the acquired image (see Tsien et al. in "Handbook of Biological Confocal Microscopy", Plenum Press, New York 1995, pages 459-478). Freely oscillating mirrors have further the disadvantage of low long-term stability. In addition to the cited problems it is a drawback of the resonance systems that operation is tied to a fixed resonant frequency which as a mechanical parameter cannot be varied and which may result in undesirable beat frequencies when superimposed by other resonant frequencies of the system.
Another problem associated with LSM mirror control is achieving sufficient phase stability when changing the direction of the motion, this being the reason why in conventional LSM in avoiding a dislocation between subsequently scanning lines half of the working cycles are dumped and only the scan for mirror motion in a predetermined direction made use of in each case for data acquisition.
Thus, the problem quite generally in LSM systems is that where mirror control involves feedback with a linear command function the operating frequency is limited and where done without feedback, although higher frequencies can be attained, this is only possible with reduced stability and at the expense of more time and processing being needed. This problem illustrated by way of LSM systems occurs in all applications in which fast periodic motion of an object having an inert mass needs to be moved in keeping with a command function. This relates to e.g. scanning reflectors moved in general, e.g. as in laser printers or in laser show systems, but also to moved scanning signal sources or the like.
It is generally appreciated that frequency-dependent amplitude and phase errors may occur in closed loops having analog amplification, this usually being counteracted by increasing the amplification factor. However, this is possible only to a limited extent since self-excited oscillations may occur in the control circuit or the amplifier may be driven into a non-linear operating range. In both cases prohibitive disturbances materialize for the controlled periodic motion, e.g. of reflector elements, due to undesirable frequency components, this in turn resulting in a restriction to low operating frequencies.